DEFLAGRATION TO DETONATION TRANSITION IN LARGE CONFINED VOLUME OF LEAN HYDROGEN-AIR MIXTURES

The results of large-scale experiments on turbulent flame propagation and transition to detonation in a confined volume of lean hydrogen-air mixtures are presented. The experiments were in a strong concrete enc losure of 480 m(3), and of 69.9 m length. The experimental volume cons ists first of a channel (34.6 m length, 2.3 m height, 2.5 m width) wit h or without obstacles, a canyon (10.556.3*2.5 m), and a final channe l. Ignition was with a weak electric spark at the beginning of the fir st channel. The effect of hydrogen concentration (9.8%-14% vol.) on tu rbulent flame propagation and transition to detonation was studied. Th e obstacle configuration in the first channel (blockage ratio 0.3, 0.6 , and no obstacles), exit cross section to the canyon (1.4, 2, and 5.6 m(2)), and vent area at the end (0, 2.5, and 4 m(2)) were varied in t he tests. Details of turbulent flame propagation, of pressure field, a nd of detonation onset are presented. A minimum of 12.5% of hydrogen w as found to be necessary for transition to detonation. This is a much less sensitive mixture than those in which the onset of spontaneous de tonation has previously been observed (minimum of 15% of hydrogen in a ir). The effect of scale on the onset conditions for spontaneous deton ation is discussed. The characteristic geometrical size of the mixture for transition to detonation is shown to be strongly related to the m ixture sensitivity.